J. Phys. Chem. C 2008, 112, 13475–13482
13475
Size Contraction in Pt/C and PtRu/C Commercial E-TEK Electrocatalysts: An in Situ X-ray Diffraction Study Walter Vogel National Central UniVersity, No. 300 Jung-Da Rd., Chung-Li, Taoyuan, Taiwan 32001 ReceiVed: April 23, 2008; ReVised Manuscript ReceiVed: June 24, 2008
High performance carbon supported Pt/C and PtRu/C (1:1) alloy electrocatalysts supplied by E-TEK are widely used as reference for fuel cell applications. A thorough in situ X-ray structural characterization of these catalysts, using a high-intensity, low-background Guinier system has been conducted. The diffraction patterns were simulated by model Debye functions after careful subtraction of support scattering and correction for the related angular factors. Size dependent contractions against the bulk lattice constants of 0.8% have been observed for both catalysts in the as received state (particle size of 2-3 nm) after previous surface oxide reduction at low temperature. The lattice contractions were alleviated after gas phase CO oxidation or the oxygen reduction reaction (ORR) at around 200 °C. Twinning defects, incorporated to the fcc-type model clusters, largely improved the fit in the case of the alloy catalyst. The Debye Function Analysis (DFA) used in this study yields the particles size distribution and provides an accurate value for the lattice constants. The average intraparticle atomic distance is a measure for the perfection of alloying in a binary system. Data from our study show that the PtRu/C particles are in a single phase state of perfect alloying, and the observed low value of the lattice constant (a ) 3.832 Å) is due to a size-related cluster contraction. Introduction A vast amount of research is focused on the improvement of fuel cells performance, and platinum-based catalysts are still the most effective anode and cathode catalysts at present. The activity and durability of the catalysts are two key factors for the fuel cell performance.1,2 High-performance Pt-based fuel cell catalysts prepared by E-TEK are widely used as a reference standard for new catalysts.3-12 Unfortunately, there still lacks detailed understanding regarding the structure, stability, and catalytic activities of this system. The majority of work published on this subject uses XRD as a routine technique, which provides basic information of the crystal symmetry and particle size by the use of the Scherrer equation. The latter is questionable, especially when particle sizes appear in the range of nanometer. The objective of this work is to obtain a detailed in-depth insight of the real catalyst structure by using both experimentally and analytically advanced X-ray diffraction methodology. This includes in situ XRD studies and experiments performed under working conditions (in opperando). The Guinier diffractometer used in this study requires a typical time of 30 to 60 min for a high-quality diffraction pattern (2θ ) 10° to 96°, X-ray tube operated at 40 kV, 20 mA). This would not be comparable with the high time resolutions provided by synchrotron sources. Studies of structural changes within a time span of ∼5 s can, however, be reached with a conventional X-ray system by using an alternative technique, which will be detailed in the Experimental Details section. It is generally accepted that the high activity of Pt-Ru bimetallic catalysts is mainly related to the bifunctional character of the alloy surface: adsorption of CO on Pt atoms and oxidative removal of CO by oxygen-like species adsorbed on adjacent Ru atoms. It has, however, frequently been reported that the surface composition of supported platinum-ruthenium nanoparticles deviates strongly from the nominal composition, with an excess of Ru atoms at the surface.13,14 X-ray absorption
spectroscopy (XAS) proved to be a powerful technique for characterization of bimetallic catalysts.2,3,14,15 X-ray absorption near edge structure (XANES) can reveal the oxidation state and d-band occupancy of a specific atom, while the local atomic structure can be obtained from the analysis of extended X-ray absorption fine structure (EXAFS). In contrast, XRD reacts on the ordering within a coherently scattering domain, averaging over all pair distances within the domains. In common with XAS, XRD is a volume-sensitive method. Surface sensitivity becomes notable for particle sizes small enough to expose a considerable fraction of atoms to the surface. The misleading term “X-ray amorphous” has often been addressed to such highly divided materials. The typical “fingerprint” in the diffraction pattern of a nanocrystal contains the information of its intrinsic “long-range order”. Supported catalyst particles can be considered as perfect “powder” material. To calculate the X-ray diffraction of such systems by use of Debye functions (DF’s) is straightforward. The DF specifies the spherical averaged Fourier transform of the atomic electron density of the respective model particle. We have used the method of Debye Function Analysis (DFA) to simulate the reduced diffraction patterns.16-21 One of the important parameters derived from DFA is the mean interatomic distance or “lattice constant”. The latter term relates to particles adopting the bulk symmetry based on a repetitive unit cell, in contrast to noncrystallographic multiply twinned particles (MTPs). As a measure for the degree of alloying of, e.g., Pt and Ru atoms the measured lattice constant can be compared with the known bulk parameters. Large deviations from the bulk parameters are expected for a core-shell structure of the bimetallic particles, depending on which species segregates to the particle surface.21 For a wellalloyed particle, however, the lattice constant should be close to the bulk value of the same nominal composition. Deviations may be attributed to (1) a composition being different from the nominal composition during preparation or (2) cluster size
10.1021/jp803527z CCC: $40.75 2008 American Chemical Society Published on Web 08/07/2008
13476 J. Phys. Chem. C, Vol. 112, No. 35, 2008
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induced intraparticle contraction. The nanoparticle size contraction effect is clearly observed for the E-TEK catalysts studied in this work. Experimental Details Samples. Two commercial E-TEK catalysts have been used for this study: 20% Pt on Vulcan XC72 (batch no. 2869600, and 20% Pt-Ru alloy on Vulcan XC72, atomic ratio ) 1:1 (batch no. 27521199). They will be designated as Pt/C and PtRu/ C. For the XRD studies they have been pressed to thin pellets 0.2 × 11.5 × 15.5 mm in size (∼26 mg). These pellets are fragile and would deform at high temperature during catalytic use. Therefore they were sandwiched between two 0.1 mm highpurity, perforated beryllium platelets before insertion to the in situ cell. The narrow beryllium peaks superimposed to the diffraction patterns can serve as reference peaks to check the correct angular peak positions. X-ray Diffraction Apparatus. A Guinier powder diffractometer (Huber), set at 45° transmission angle, was used in this work. A Johansson-type Ge monochromator produces a focused high-intensity monochromatic Cu Ka1 primary beam. The samples were fixed between two 3 mm polyethylene foils and measured while under ambient conditions. Most of the experiments in this work were conducted by a specially designed in situ cell described elsewhere.17,22 The patterns were subjected to angular dependent corrections for the absorption of the sample pellet, the polarization factor, modified by reflection on the crystal monochromator, and a geometrical factor related to the specific Guinier geometry. Scaling the peak intensity of the pure substrate pattern to that of the supported catalysts and then subtracting this background contribution yields the diffracted intensity produced by the catalytically active metal phase. Vulcan XC72 carbon gives a broad 002 reflection, which can be used as a reference peak for the background calibration. Note that the position of the incident beam and the sample is fixed during prolonged in situ treatments. This has the important consequence that the resulting diffraction patterns can be compared on an absolute scale (such as those in Figure 8). For the in situ study of dynamical processes we have used the “Open Slit” (OS) technique: the diffraction angle of the detector arm will be fixed to the peak position of a strong Bragg peak with the receiving slit widely open (∆θ ) 1°). The count rate collected this way is sufficiently accurate for a 5 to 10 s time resolution. This signal has been shown to be highly sensitive to some surface oxide formation/reduction,19,22-24 and also serves as an indicator for particle growth.20,25 Debye Function Analysis (DFA). Numerical simulation with the help of the Debye function analysis (DFA) yields information on the intrinsic structure of the colloids, including the average “lattice constant”, and the size distribution of the assembly assuming coherently scattering particles within the catalysts.16,18,20,25 In all cases homogeneous alloys should, however, yield a diffraction curve that can be simulated by using the average atomic scattering amplitude. At its simplest level, the DFA method is based on the addition of DF’s of a sequence of model clusters with increasing size. The sum is then compared with the experimental intensity. The precalculated DF’s are stored in a separate input file. These model clusters could for instance be shell-wise increasing cuboctahedra, having so-called magic numbers nuclearity (N ) 13, 55, 147, 309, ...). DFA then uses a set of free parameters for the number fractions of the individual clusters to fit the experimental data. In a pevious paper Hall has given a detailed evaluation of this method.26 The present study uses the Levenberg-Marquardt (L-M) algorithm
Figure 1. XRD scan of Pt/C E-TEK catalyst in vacuum at room temperature (red line) and the scaled scan of the Vulcan XC-72 support (black). The dotted bar marks the “Bragg window” at the Pt-111 peak. The asterisks indicate beryllium peaks of the sample holder.
for the numerical simulations. Thermal and static local displacements are incorporated via a Debye-Waller factor. This factor affects a progressive dampening of the peak intensities, likewise adding a diffuse scattering to the DF’s. A scaling factor for the abscissa in reciprocal b-space (2sin(θ)/λ, 2θ is the scattering angle; λ ) 1.5406 Å, wavelength) allows for an expansion/ compression of the DF’s. For the precalculated DF’s a fixed lattice constant is used (3.9236 Å for bulk platinum). By this method an accurate determination ((0.003 Å deduced from the confidence limits of the L-M algorithm) of the average lattice spacing (contraction/expansion against bulk Pt) within the particles is achieved. Lattice constants derived from single diffraction peaks are inherently unreliable due to peak overlap and the unknown baseline. Moreover fcc-type stacking faults, which are theoretically predicted and frequently observed in metal nanoparticles,20,27,28 will produce small shifts of the peak positions according to Warren’s basic work.29 A discussion of the limitations of DFA including confidence limits and the effect of an inappropriate choice of model functions will be found in the Appendix. A reliability factor R, defined as relative standard deviation, will be included to the figures of all DFA simulations:
R)
1 N
N
(yexp2 - ycalc2)
1
yexp2
∑
Results and Discussion E-TEK Pt/Vulcan XC72. A considerable amount of surface oxide is usually formed at highly dispersed metallic catalysts even after short storage in air. The presence of surface oxide has been evidenced by previous studies, e.g., platinum on silica standard catalyst EuroPt-1,16 and for a bimetallic PtRu/SiO2 catalyst.21 In situ X-ray diffraction can in fact detect and also perform a controlled removal of surface oxide by low temperature hydrogen reduction. Here we have applied short pulses of H2 (∼1 mL) to the catalysts, while holding the system under vacuum at room temperature. The reduction was monitored by the OS method, described in the Experimental Details section. Figures 1 and 2 demonstrate the use of this method: Figure 1 shows the XRD scan of the E-TEK Pt/C catalyst. The dashed bar is the position of the “Bragg window” with the receiving slit placed at the strongest Pt-111 Bragg peak. The effect of H2 pulses on this signal intensity (I-signal) is shown in Figure 2. At the beginning the cell is vented. Evacuation produces a slight increase of I-signal, which is solely related to the removal of air, since the X-ray beam is attenuated by air absorption. Three H2 pulses are applied, for each incident the sample surface
Pt/C and PtRu/C Commercial E-TEK Electrocatalysts
Figure 2. E-TEK catalyst Pt/C as received. Reduction of surface oxide with H2 pulses at room temperature. Red line: Pt-111 intensity. Green line: sample surface temperature.
Figure 3. XRD scans of Pt/C (dots): as received, reduced in H2 (black), and after catalytic use at 220 °C, and finally reduced in CO (red, plotted with an offset of 500 c/s). Both scans have been measured at room temperature under vacuum. The solid lines are the DFA simulations.
temperature shows a positive spike, while the I-signal shows a stepwise increase, simultaneously. Only two pulses are needed to fully reduce the platinum surface. A third pulse does not induce further reduction. The time resolution is 5 s, and surface reduction proceeds within this time frame. After venting the cell a down-step of the I-signal is again produced by air absorption. The further lowering of the I-signal is likely related to a slow reoxidation of the platinum surface. We have conducted in operando studies of CO oxidation as well the oxygen reduction reaction in an H2/O2 gas flow at low and elevated temperatures using the OS method. These results will be published in a forthcoming paper. The interest of this work is focused on the accurate evaluation of structural parameters available by advanced XRD, and modifications of the initial prereduced state to the final state after catalytic use. Figure 3 shows the diffraction pattern of Pt/C before and after catalytic use. The experimental data (dotted lines) are superimposed with the simulated diffraction patterns using DFA (solid lines). For these simulations we used regular fcc-type model clusters PtN with cuboctahedral morphology (N ) 309, 561, 923, 1415, ..., 14975). The biggest model cluster in this series is a 16-shell cuboctahedron with a sphere-equivalent diameter of 7.6 nm. During catalytic CO oxidation this sample was exposed to 220 °C for about 3 h. Figure 4 summarizes the size distributions weighted by number (left panel), or by the particle mass (right panel) for Pt/C catalysts before and after CO oxidation. As evidenced from the panels in Figure 4 the E-TEK Pt/C catalyst displayed a bimodal size distribution. The number average diameter 〈DN〉 increases from 2.3 to 2.8 nm, while the
J. Phys. Chem. C, Vol. 112, No. 35, 2008 13477
Figure 4. Pt/C size distribution: as received (black) and after catalytic use (red).
TABLE 1: Lattice Constants of E-TEK Catalysts at Room Temperature E-TEK a (Å) manual catalyst from DFA estimatea
sequential pretreatment
Pt/C
3.892
H2 pulses at rt
Pt/C
3.923
PtRu/C
3.832
PtRu/C
3.839
3.912 3.840 3.837 3.841
PtRu/C
3.850
3.842 3.850
3.894 3.896 3.914
CO oxidation, 220 °C, 2 h
mass-weighted mean size (nm) 3.1 4.7
H2 pulses at rt
2.7
CO oxidation, 220 °C, 45 min
3.0
N2, 30 min, 650 °C
4.5
3.848 a
Estimated from the peak positions of reflections 220 and 311.
mass average diameter 〈DM〉 increases from 3.1 to 4.7 nm after CO oxidation. Interestingly, both large and small particles show an increase of diameter after reaction, but the bimodal feature remains unchanged. DFA has a tendency to overestimate the smallest particles; therefore the mass-averaged diameter is regarded as a more representative number. The lattice constants from DFA simulations are a ) 3.892(3) Å after hydrogen pretreatment at room temperature. This number increases to a ) 3.923(3) Å, adopting the value of bulk Pt after catalytic use (see Table 1). E-TEK PtRu/Vulcan XC72. Surface oxide reduction of the binary PtRu alloy particles by hydrogen is more retarded, compared to the Pt/C catalyst. Figure 5 shows the same OS treatment as in Figure 2. Not surprisingly, more H2 pulses are required for a full reduction. And for each incidence, the increase I-signal is less pronounced compared with that in Pt/C. The sample surface temperature also increases simultaneously at the incident of hydrogen pulse, but decays relatively slowly compared to the case in Pt/C catalyst. Ruthenium atoms exposed to the particle surface are easily oxidized in air. Pure Ru nanoparticles have been found to be fully converted into an amorphous RuOx species after storage in ambient conditions for about 1 month.24 Most likely RuOx species on the surface will exceed those of PtOx. Reduction of an oxide overlayer formed on PtRu alloy particles should therefore produce a ruthenium-enriched surface. In this context it is important to know the fraction of metal that has been transformed to oxide. Indeed, a quantitative number for this fraction can be found from the relative change of the Bragg peak intensity, which for
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Figure 5. E-TEK catalyst PtRu/C as received. Reduction of surface oxide with H2 pulses at room temperature. Red line: Pt-111 intensity. Green line: sample surface temperature.
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Figure 7. Relative amount of metal reduced to Me0 by consecutive H2 pulses at room temperature, deduced from the increase of the Pt111 Bragg peak intensity.
Figure 6. Model explaining the effect of an amorphous surface oxide layer on the intensity of a Bragg peak.
X-rays is proportional to the volume of the coherently scattering domain. This volume will be less if some of the atoms rearrange at the surface to form an amorphous oxide layer. As a supposition, the total scattering volume of the sample must be kept unchanged during such an experiment. A reasonable model for surface oxidation/reduction implies (1) oxide is only formed on the surface of the respective particle and (2) after reduction the zerovalent metal atoms will sort in registry with the underlying nanocrystal lattice. The schematic picture in Figure 6 should give a better understanding of this process. The proposed mechanism is supported by (1) the observed ultrafast reduction time and (2) the reversibility of the oxidation. The latter has been proved by in situ XRD for the standard Pt/SiO2 catalyst EuroPt-1. Over many H2/O2/H2 cycles at 300 °C the catalyst was shown to return to its original state with the same particle size distribution.16 According to this model, the fraction being oxidized would be equal to the relative change ∆I/I0 of the I-signal, with I0 being the metal Bragg peak intensity measured in the reduced state. I0 must be corrected for all contribution to the background including the carbon support scattering at the specific angle (compare Figure 1). For the E-TEK catalysts, the fraction oxidized thus found is 12 wt % for Pt/C and ∼25 wt % for PtRu/C (Figure 7). This rather high amount of surface oxide is also evidenced from the diffraction pattern, measured in situ, before and after H2 reduction (some gaps are seen in the patterns, which are related to the cutouts of the strong Be peaks; Figure 8). Between θ ) 10° and 18° an excess intensity is seen, which is typically produced by amorphous RuOx oxides.24 Note that the use of the simple Scherrer equation for crystallite size
Figure 8. In situ XRD pattern of the PtRu/C catalyst. As received (red) and reduced in H2 at room temperature (black).
determination would necessarily lead to an erroneous (too small) particle size. The present results stress the importance of performing X-ray diffraction under in situ conditions, and the use of more advanced numerical methods such as DFA for data evaluations. The DFA simulations of the prereduced PtRu/C catalyst shown in Figure 9 were performed with three different sets of model particles: (a) using regular fcc particles, (b) using decahedral model particles, and (c) using fcc model clusters containing one twin fault. From simple visual inspection the twinned particles give the superior fit, showing virtually no difference between experiment and simulation. The indicated R-values manifest this conclusion. Twinning is known to produce an inward shift of the two first reflections 111 and 200, making them become closer to each other. This is evidenced by inspection of curve a in Figure 9. The insert shows the number and mass averaged size distribution used for the twinfaulted DFA simulation. Compared to the E-TEK Pt/C catalyst the size is confined within a narrow range. The result is in good agreement with the TEM size (2.5-3.5 nm) of this E-TEK catalyst observed by Liceano et al.5 A consecutive annealing at 650 °C in nitrogen induces a further growth to an average size of 4.5 nm (Figure 10). The lattice constants of both catalysts found by DFA are shown in Figure 11. The published lattice parameters of the
Pt/C and PtRu/C Commercial E-TEK Electrocatalysts
Figure 9. E-TEK catalyst PtRu/C XRD patterns are simulated by DFA after H2 reduction at room temperature [experimental data (dots) and calculated data (solid lines)]: (a) perfect fcc cuboctahedra, (b) decahedra, and (c) twinned fcc model particles. The insert shows the size distribution weighted by mass (black bars) and by number (red bars) for the twin-faulted models.
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Figure 12. Model calculation showing the effect of the intermixing/ segregation on the Debye function of a 309-atom cuboctahedron (dark balls, Pt; light balls, Ru). For more clarity, models B and C are shown as half-sections.
Figure 10. E-TEK catalyst PtRu/C after high temperature treatment at 650 °C in N2 [(+) experimental data and calculated data (solid lines)]. Simulation by DFA with single twin-faulted fcc-type model particles. The insert shows the size distribution. Figure 13. E-TEK PtRu/C background correction to DFA. The diffuse background (blue line, dash-dotted) is calculated by using the fit parameter B ) 1.2 Å2. The curve shown with offset is the fit with the constrained parameter B ) 0.
Figure 11. Bulk lattice constant of the Pt-Ru system (circles). The DFA results for the E-TEK nanoparticles are inserted to the diagram: black squares, PtRu/C; red squares, Pt/C.
bulk Pt-Ru system, prepared by arc melting, are included in the figure.30-32 Both E-TEK catalysts, with surface oxide being removed by ambient temperature H2 reduction, exhibit a lattice
contraction of 0.80% against the bulk. We refer this contraction to a size related quantum size effect, in rather good agreement with published literature for platinum.20,33,34 In a droplet model this trend would be explained by surface tension, which compresses the clusters more strongly as the surface-to-volume ratio increases. From a related viewpoint, one might consider the reduced coordination numbers of the surface atoms as the source of the contraction: the valence electrons are distributed among a smaller number of bonds.35 Recently Mott et al. have suggested a size-induced contraction for the nanoscale goldplatinum alloy36 system from XRD data. Their conclusions are based on a DCP-AES analysis of the chemical composition, which differs from the nominal composition of the synthesis.37 However, the authors could not confirm a contraction for the pure metal particles Au and Pt with the same particle sizes around 4 nm, as compared to their bulk counterpart. After CO oxidation at 220 °C the pure platinum Pt/C particles grow from 3.1 to 4.7 nm, and the lattice constant approaches that of bulk platinum. In contrast, the PtRu/C catalyst alloy particles are more stable, and grow only slightly after catalytic use for CO oxidation. The effect on the lattice constant is less pronounced (Figure 11). Only at temperatures as high as 650
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Figure 14. E-TEK Pt/C size distributions calculated with regular fcc model particles, but with different input parameters to the L-M routine (see text).
TABLE 2: The L-M Parameters for Fit (a) of Figure 14 and the Related Confidence Limits free parameter no.
fitted parameter
lower confidence
upper confidence
4 5 6 7 8 9 10 11 33 34 36 37 38
2.25497 0.730758 0.0695093 0.0660373 0.0333414 0.327405 1.0139 0.214451 -97.206 -0.0080854 0.64446 2.9729 97.824
1.99266 0.476318 0 0 0 0.172557 0.840308 0 -155.789 -0.00980585 0.31665 1.9173 60.23
2.52434 0.990403 0.320219 0.305162 0.228014 0.514059 1.19418 0.29577 -39.5539 -0.00636491 1.52552 4.26187 132.398
°C does the lattice contraction reduce to 0.34% with respect to the bulk alloy. Objections to the above conclusions could be raised if the Pt-Ru alloy composition would distinctively deviate from E-TEK’s nominal 50 atom % composition. According to Figure 11 the fraction of Ru atoms must be about ∼70 atom % to fit with the bulk data. A large deviation from the nominal 50 atom % can be excluded. Roth et al.38 have found the composition of this E-TEK catalyst to be close to 50 atom % by X-ray fluorescence analysis. Sivakumar et al. 9 found 50.2% Pt/49.8% Ru by averaging a large number of EDX measurements. Wang and Hsing11 reported XPS results of similar 1:1 ratio for the E-TEK PtRu/C catalyst. Moreover, the solubility of Ru in Pt is limited to ∼60 atom %. At high Ru fractions a separation into an hcp-type phase rich in ruthenium plus a Pt-Ru alloy phase will appear. This was frequently reported for homemade Pt-Ru catalysts,13,21,38 and becomes visible by the appearance of extra peaks corresponding to a hcp-ruthenium phase. Phase separation is preceded by a surface enriched in ruthenium. The E-TEK PtRu/C catalyst did not, however, show a phase separation after heating to 650 °C in N2. Roth et al. have heat-treated this catalyst up to 1200 °C, without any evidence of Ru reflections, backing the supposition that the commercially available catalyst forms a true alloy.38 To address the size mismatch effect, a simplified model calculation has been performed in an earlier paper:21 Suppose
Vogel a cuboctahedral Pt-Ru cluster, atomic ratio close to a 1:1 mixture, containing 309 atoms. This cluster can adopt three extreme conformations (Figure 12, from left to right): (A) a random distribution of the two species on lattice sites, with the “lattice constant” ) 3.863 Å, equal to the bulk PtRu alloy; (B) a shell of 162 Pt atoms on the surface, and a core of 147 Ru atoms; and (C) a shell of 162 Ru atoms on the surface, and a core of 147 Pt atoms. Here we assume that the core atoms adopt the bulk interatomic distance, and the outer shell is slightly (3%) outward expanded for the Pt shell, and 3% inward contracted for the Ru shell, respectively. This model should roughly account for the size mismatch of the atomic radii. The related DF’s marked as A, B, and C in Figure 12 do not show a major overall change, except a certain expansion/contraction in the b-scaling of model B and C in comparison to model A (solid line) with random site occupation. The average “lattice constants” derived from DF’s B and C are 3.81 and 3.90 Å, respectively. Intracluster segregation should therefore show up as a distinct deviation from the bulk bimetallic lattice constant. Apparently the value observed by diffraction data is dominated by the distance of the atoms forming the nanoparticle core. Our present results on the E-TEK PtRu/C catalyst could therefore be in favor of a core enriched with ruthenium. As mentioned above, the reduction of preferentially RuOx surface species would, however, produce a surface enriched with ruthenium. EXAFS studies of Hwang and co-worker15 and Liu et al.14 show, in fact, that PtRu E-TEK catalyst nanoparticle surfaces are enriched in ruthenium after reduction at room temperature. It is worth noting that the above EXAFS studies used E-TEK catalysts with a higher loading of 30 wt %. Antolini has shown that the amount of metal loading has an impact on the nanoparticles structure.39 On the basis of the previous model calculation a Ru-rich shell will be of little effect on the lattice constant determined by XRD of an otherwise well-alloyed core. It is therefore believed that the low lattice constant must be related to a size-induced lattice contraction of the Pt-Ru alloy particles. Conclusion We conclude from the above arguments that the binary PtRu/C E-TEK catalyst nanoparticles are in a single phase state of perfect alloying, and the observed lattice contraction of 0.80% against the bulk counterpart is a quantum size effect. A sequential lowering of the contraction to 0.34% after a thermally induced growth of the particles evidence the size dependence. To the best of our knowledge, this has been reported the first time for alloy nanoparticles. Support comes from the pure platinum E-TEK catalyst of about the similar particle size of 2-3 nm, for which we have observed the same lattice contraction of 0.80%. The stability of the alloyed catalyst PtRu/C under similar conditions of CO oxidation at 220 °C is higher than that of the Pt/C catalyst. The present study proposed a convenient approach through in situ XRD experiment facility and detailed DFA analysis of the nanocrystalline catalysts material to evaluate the change of catalysts structure characteristics (lattice constants, surface oxidation, degree of alloying) during catalysts operation. Appendix The Levenberg-Marquardt method has proved of great value for DFA. Here we use the original coding by Baumeister and Marquardt, which also provides the confidence limits. The iterative nonlinear least-squares routine quickly converges to a
Pt/C and PtRu/C Commercial E-TEK Electrocatalysts global minimum (typically 10 iterations). It is rather independent of the choice of the starting parameters. In fact starting with all-zero parameters is usually sufficient. In using the Debye function analysis the background correction needs special attention. In the present study background correction is quite accurate (compare Figure 1), but a small linear correction term (two parameters) is still needed. A physical contribution to the background comes from thermal diffuse scattering and local static displacements,16 which adds a diffuse term to the DF’s. In turn the peak intensities of the DF’s will bedampedwithincreasingb-valuesbyageneralizedDebye-Waller factor exp(-2M). It includes local static displacements, with M ) Bb2/4 and B ) Bstat + Btemp(T). B(T) is used as a free parameter, and its numerical fit values are usually of the order of 1-3 Å2. Not surprisingly they are distinctively larger than the common bulk Debye parameters Btemp(T) (Btemp(T) ) 0.22 Å2 for platinum at 20 °C). Since the present DFA study uses geometric model clusters, the large B-parameters account for, e.g., surface reconstruction due to surface oxide or chemisorbed species. Chemical bonds to a supporting substrate or to a protective ligand shell will also affect the local surface geometry. Unfortunately the confidence limits of DFA are in general large for the B-parameter. It is therefore only in special cases possible to retrieve unambiguous conclusions related to this number.16 EXAFS provides the more accurate B-values, since it probes only the local environment of the atoms, but their interpretation is different. Figure 13 shows the fitted background in the case of the alloy catalyst E-TEK PtRu/C, reduced at room temperature (black line). The blue line (dash dotted) is the contribution of diffuse scattering due to disorder, with B ) 1.21 Å2. For this alloy catalyst only 4 DF’s were needed (twinned fcc particles, sphere-equivalent diameters between 2.1 and 3.5 nm). The R-value is as low as 0.69%. The total number of free parameters used for this fit is N ) 8 (4 weight factors for the DF’s, 2 for a linear background, 1 for the B-parameter, 1 for the lattice parameter). The addition of 4 larger DF’s with diameters up to 4.9 nm did not improve the fit. Since the Debye functions are calculated from pair distances, and since the real particles are confined to a small size, inclusion of larger particles should in fact not improve the simulation. For comparison purposes, the curve plotted with an offset in Figure 13 demonstrates the DFA best fit with a B-parameter constrained to a zero value. Figure 14 demonstrates the influence of the use of different starting parameters in the L-M routine, exemplified for the monometallic catalyst E-TEK Pt/C, reduced in H2 at room temperature. This catalyst has a wider size distribution. We have used geometrical cuboctahedra as model clusters with undistorted fcc symmetry and sizes starting from Pt309 (4 shells) to Pt5083 (11 shells) for the fitting. For a wide size distribution it is reasonable to form two groups of particles, three smaller clusters with 4 to 6 shells, and a group of five with 7 to 11 shells, respectively. Each group will have its own independent Debye parameter B. A total of 13 free parameters are then used for this simulation. All of them were set to zero as input to the L-M routine. Line a in Figure 14 shows this fit. The mass fractions are plotted versus the sphere equivalent diameters. The number of the two B-parameter is added to the figure. Line b is a fit starting with the final parameters of line a, but uses only one single B-parameter in common to all model particles. Line c is the same fit as line b, but uses all zeros as input to the L-M routine.
J. Phys. Chem. C, Vol. 112, No. 35, 2008 13481 Line d is the same fit as line c, but uses the final parameters of line c as input. It is evident from the graph that the final results are not too far from each other. Particularly the bimodal feature of the size distribution is persistent. The L-M routine converges to a minimum with similar output parameters, independent of the choice of the input parameter. The R-values are similar for all 4 simulations. It is worthy of mention that the Debye parameter resulting from fit a is large (3.04 Å2) for the small particle fraction, and much smaller for the model particles Ptn, with n g 1415 (0.63 Å2). (The subscripts are error limits, about 1/3 of the confidence limits.) This is reasonable, since the overall effect of surface disordering should be smaller for the larger particles. If we do not split into two groups, the overall value is B ) 1.82 Å2 for fits b, c, and d. Table 2 contains the fitting parameters and the related confidence limits of fit a in Figure 14. Parameters 4 to 11 are the relative frequency by mass of 4-shell to 11-shell cuboctahedra; parameter 33 and 38 are gradient and start values of a linear background correction term; parameter 34 is the relative change of the lattice constant with respect to bulk platinum; and parameter 36 and 37 are the Debye parameters for the large fraction and small fraction of model particles, respectively. Acknowledgment. The financial support from the National Science Council (under contract number NSC96-2113-M-008008) and the National Central University, Taiwan, ROC, is gratefully acknowledged. References and Notes (1) Borup, R.; Davey, J.; Wood, D.; Garzon, F.; Inbody, M.; Guidry, D. PEM Fuel Cell Durability, 2005 DOE Hydrogen Program Review; Los Alamos National Laboratory, 2005. (2) Sarma, L. S.; Chen, C.-H.; Wang, G.-R.; Hsueh, K.-L.; Huang, C.-P.; Sheu, H.-S.; Liu, D.-G.; Lee, J.-F.; Hwang, B.-J. J. Power Sources 2007, 167, 358. (3) Hannemann, S.; Casapu, M.; Grunwaldt, J.-D.; Haider, P.; Truessel, P.; Baiker, A.; Welter, E. J. Synchrotron Radiat. 2007, 14, 345. (4) Lizcano-Valbuena, W. H.; Paganin, V. A.; Gonzalez, E. R. Electrochim. Acta 2002, 47, 3715. (5) Lizcano-Valbuena, W. H.; Paganin, V. A.; Leite, C. A. P.; Galembeck, F.; Gonzalez, E. R. Electrochim. Acta 2003, 48, 3869. (6) Pozio, A.; Silva, R.; Francesco, M. D.; Cardellini, F.; Giorgi, L. Electrochim. Acta 2002, 48, 255. (7) Raman, R.; Shukla, A.; Gayen, A.; Hegde, M.; Priolkar, K.; Sarode, P.; Emurac, S. J. Power Sources 2006, 157, 45. (8) Santos, L. d.; Colmati, F.; Gonzalez, E. R. J. Power Sources 2006, 159, 869. (9) Sivakumar, P.; Ishak, R.; Tricoli, V. Electrochim. Acta 2005, 50, 3312. (10) Tada, M.; Murata, S.; Asakoka, T.; Hiroshima, K.; Okumura, K.; Tanida, H.; Uruga, T.; Nakanishi, H.; Matsumoto, S.-I.; Inada, Y.; Nomura, M.; Iwasawa, Y. Angew. Chem., Int. Ed. 2007, 46, 4310. (11) Wang, X.; Hsing, I.-M. Electrochim. Acta 2002, 47, 6. (12) Xiong, L.; Manthiram, A. Solid State Ionics 2005, 176, 385. (13) Antolini, E. Mater. Chem. Phys. 2003, 78, 563. (14) Liu, D.-G.; Lee, J.-F.; Tang, M.-T. J. Mol. Catal. A 2005, 240, 197. (15) Chen, J.-M.; Sarma, L. S.; Chen, C.-H.; Cheng, M.-Y.; Shih, S.C.; Wang, G.-R.; Liu, D.-G.; Lee, J.-F.; Tang, M.-T.; Hwang, B.-J. J. Power Sources 2006, 159, 29. (16) Gnutzmann, V.; Vogel, W. J. Phys. Chem. 1990, 94, 4991. (17) Hartmann, N.; Imbihl, R.; Vogel, W. Catal. Lett. 1994, 28, 373. (18) Vogel, W. Cryst. Res. Technol. 1998, 33, 1141. (19) Vogel, W.; Alonso-Vante, N. J. Catal. 2005, 232, 395. (20) Vogel, W.; Bradley, J.; Vollmer, O.; Abraham, I. J. Phys. Chem. B 1998, 102, 10853. (21) Vogel, W.; Britz, P.; Bonnemann, H.; Rothe, J.; Hormes, J. J. Phys. Chem. B 1997, 101, 11029. (22) Hartmann, N. Zur oszillatorischen Dynamik der an Platinmetallen katalysierten Oxidation von Kohlenmonoxid mit Sauerstoff und Stickstoffmonoxid, Freie Universita¨t: Berlin, Germany, 1997.
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